Here is a great web-link for an easy approach to learning serial dilutions, and how to apply them to microbiological practices...


Serial Dilutions Made Easy
Author: Jan Hilten and Carol Sanders
Woodrow Wilson Biology Institute
1993

Introduction:

Many areas of science use serial dilutions in the preparations for different experiments. This exercise is presented as an aid for the teacher in helping his/her students improve their skills and more quickly understand the particular application for which serial dilutions are a tool. Serial dilutions are often used in microbiology, biotechnology, and in chemistry classes, to name just a few. Therefore a clear, concise, and nonthreatening approach to the learning of this very important concept is essential.

Serial dilutions are usually made in increments of 1000, 100 or 10. The con-centration of the original solution and the desired concentration will determine how great the dilutions need to be and how many dilutions are required. Important also is the total volume of solution needed. If only small quantities of solutions are needed then greater numbers of dilutions are necessary.

The most common examples deal with concentration of cells or organisms, or the concentration of a solute. The approximate concentration should be known at the start of the experiment before the appropriate number and amount of dilutions can be made. In order to arrive at the desired concentration, use serial dilutions, instead of making one big dilution, in order to finally arrive at the desired concentration. This method is not only cost effective but it also allows for small aliquots to be diluted instead of unnecessarily large quantities of materials.

This technique involves the removal of a small amount of an original solution to another container which is then brought up to the original volume using the required buffer or water. In the example below, if you have 1 mL of your original solution, and you remove 10 L and place it in a tube containing 990 L of water or media you have made a 1:100 dilution. If the original solution contained 5 x 10^8 organisms or cells/mL, we now have a concentration of 5 x 10^6 cells/mL, because we have simply divided our concentration by 100. Now, if we want to dilute this by a factor of 1:1000, we must remove 1 L of the second solution and place it in a tube containing 999 L of media. We have now diluted our secondary concentration by 1000, and would then divide our concentration by 1000 to yield a 5 x 10^3 cells/mL.

For excellent examples to help you apply serial dilutions to preparing cell suspensions LINK HERE!

Link Reference - Originally posted at The National Health Museum's Access Excellence Activities Exchange